This invention relates generally to particle study devices and is more particularly concerned with means for clearing the aperture employed in such devices.
A particle study device, embodying the Coulter principle such as is shown and described in U.S. Pat. No. 2,656,508, now is well known in the art. Wallace H. Coulter, the patentee of the noted patent, is the same person as the applicant herein. In the Coulter type particle study device, particles of microscopic material are suspended in a fluid whose electrical impedance is different from the electrical impedance of the particles. The fluid is allowed to pass through a microscopic aperture formed in an insulating wall. Electrical excitation in the form of an electric current is applied to the fluid suspension in the aperture, usually by means of electrodes immersed in the fluid suspension on opposite sides of the wall. Due to the dimensions involved, the particles suspended in the fluid flow through the aperture at a very rapid rate and each time a particle passes through the aperture there is a change in the total impedance of the fluid path which is effectively included in the aperture. This change in total impedance coacting with the electrical excitation causes a particle pulse to be developed which is used to count and size the particles passing through the aperture.
In the art of counting and sizing minute particles by used of the Coulter type device, normally the range of particles at its limit is well within the physical diameter of the aperture employed. Experience has shown, however, that even the most specialized suspension of particles will at times produce a blockage, clogging the aperture. Blockages may also be caused by lint, dirt and other debris. Anything which will partially or fully obstruct the aperture will hereinafter be called debris, irrespective of whether it is of the same particulate matter as the particles being studied or whether it is foreign matter.
Circuitry is available for detecting the blockage of the aperture in a Coulter type particle study device. One such debris detection circuit is shown and described in U.S. Pat. No. 3,259,891. Wallace H. Coulter, a co-patentee of this patent is the same person as the applicant herein. In U.S. Pat. No. 3,259,891, debris clearing devices also are shown and described. Certain ones of the devices shown and described require either complex mechanical linkages in order to remove the aperture debris, or the actual removal of the aperture and/or aperture tube. In the former case, the mechanical linkages are cumbersome and somewhat difficult to use. In the latter case removing, cleaning and replacing an aperture tube is a time consuming procedure which is to be avoided. One of the devices shown employs a capacitor charged to a high potential which is discharged via the electrodes creating a very high current flow through the aperture, thereby literally heating the contents of the aperture to explode, driving the obstruction out of the aperture. However, the rate of application of energy from the capacitor is not optimum and when sufficient energy is used to clear a blockage it creates a serious threat of damage to the aperture structure.